Why Some Rope Knots Hold Better Than Others We know that some knots hold better than others, but the why of it hasn't been so clear. Special fibers that change color under strain are providing some answers.
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A Knotty Problem Solved

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A Knotty Problem Solved

A Knotty Problem Solved

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AILSA CHANG, HOST:

Knots have been used by people since ancient times. There are thousands of different kinds of knots, everything from simple knots used to tie shoes to the far more specialized ones used by, say, mountaineers. Scientists have now come up with some basic rules that can be used to predict how a knot will perform. NPR's Nell Greenfieldboyce reports.

NELL GREENFIELDBOYCE, BYLINE: There's a whole field of mathematics that studies knots theoretically to explore abstract properties of idealized curves.

VISHAL PATIL: But that's not what you care about if you're, for example, a sailor or a climber and you need to tie something which holds.

GREENFIELDBOYCE: Vishal Patil is a graduate student at MIT. He's interested in the mechanics of real-world knots. He says, take two simple knots - the reef knot and the granny knot. They look very similar but behave very differently.

PATIL: So if you pull on the reef knot, it tends to hold. And if you pull on the granny know, it tends to slip quite easily.

GREENFIELDBOYCE: He says there's been no theoretical framework, no set of rules to explain why these knots do what they do. But then he heard of something new, a special kind of fiber developed by colleagues at MIT. This fiber changes color when it's under strain.

PATIL: And we thought that this was a good opportunity to learn more about real physical knots using their system.

GREENFIELDBOYCE: The research team tied a couple of different knots in this fiber, and the color changes let them see how forces and strain were distributed inside the knots. They used this information to fine tune some computer simulations of knots. After that, they took what they'd learned and came up with a system for identifying strong knots and weak knots.

Patil says one key thing - the thing that separates the reef knot from the granny knot - is twist.

PATIL: Twist is quite important in how knots behave.

GREENFIELDBOYCE: He says when you have lots of twists going in opposite directions along the knot, it locks the knot.

PATIL: But if lots of twists are going in the same direction, then the whole thing can roll out.

GREENFIELDBOYCE: Other key features include the amount of friction in the knot and its overall complexity. To see how well their system could predict a knot's behavior, they took six different knots and assessed how stable they should be. Then they tested that experimentally.

PATIL: We tried tying different knots and loading them with weight and seeing how much weight we needed to get them to slip.

GREENFIELDBOYCE: Their predictions were right. The research is reported in the journal Science, and it could someday be used to design new kinds of knots that are custom-made for certain jobs.

Nell Greenfieldboyce, NPR News.

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